Plasma-assisted surface modification of polymers for medical device applications

ABSTRACT

The surface of a high molecular weight polymer such as high molecular weight polyethylene is modified in a localized manner by treatment with a plasma gas. The treatment produces a variety of useful results, depending on the gas used and the treatment conditions. One such result is crosslinking of the polymer in a localized manner at the surface to improve the durability of the surface against detrimental processes such as reorientation and alignment of the crystalline lamellae parallel to the contact surface which renders the surface susceptible to disintegration into particles. Another result is the chemical transformation of the surface for purposes such as increasing the hydrophilic or hydrophobic nature of the surface or coupling functional groups to the surface.

CROSS-REFERENCE TO RELATED APPLICATION

This appln claims benefit of Prov. No. 60/110,188 filed Nov. 30, 1998.

STATEMENT OF GOVERNMENT RIGHTS TO INVENTION DUE TO FEDERALLY SPONSORSHIP

The invention was made with Government support under Grant (Contract)No. N00014-98-1-0633 awarded by the Office of Naval Research, Grant(contract) No. CMS-924978 awarded by the National Science Foundation,and Grant (Contract) No. CMS-0085156 awarded by the National ScienceFoundation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention lies in the field of medical devices such as artificialknee and hip joints. In particular, this invention addresses mattersassociated with the use of ultra high molecular weight polyethylene(UHMWPE) and other biologically compatible polymers suitable for use inthe manufacture of such devices.

2. Background of the Invention

Polymers are widely used as materials of construction in medical devicessuch as artificial joints, bio-instruments, and other medical implants.Knee joint replacements, many of which use UHMWPE as the tibiacomponent, are examples of such devices. Unfortunately, artificial kneesand other articulated implants have a limited life span in the bodysince the wear on the UHMWPE component causes the material todeteriorate and to form debris which leads to inflammation andosteolysis. Other factors that limit the life span of UHMWPE and otherpolymers used in these devices are cyclic damage, contact stresses,friction, and possibly the hydrophilic/hydrophobic character which mayaffect biocompatibility. Whatever the cause, the device ultimatelyreaches the end of its life span and a replacement is needed.Unfortunately, replacement surgery (which is termed a “revision”operation) is often more difficult and poses a higher risk than theoriginal implantation surgery. Nearly 500,000 artificial joints areimplanted in the United States each year, and the average artificialjoint lasts about 15 years before it must be replaced. This time spansuggests that a single implantation may be suitable for older or lessactive patients. Young, active patients however may require one or morerevisions, and the number of revisions increases with the increase inthe life expectancy of the general population. Aware of the low successrate of revisions, many younger patients wait (often in pain) beforetheir first arthroplasty operation in order to lessen the number ofrevisions that they will need during their lifetime.

The development of sophisticated techniques such as transmissionelectron microscopy for characterizing surfaces has led to an improvedunderstanding of wear mechanisms. As the polymer is subjected to wear,the polymer delaminates and particles of the polymer separate from thecomponent. The separated particles are then released into thesurrounding tissue. Crystalline lamellae that are part of the polymerstructure are particularly susceptible to the shear forces that arisewhen the contacting surfaces slide against each other, since these shearforces cause the lamellae to align at the surface, which increases theirsusceptibility to breakage. This causes further particle formation andseparation.

Several theories have been advanced to explain the mechanisms by whichwear occurs in the UHMWPE used in total joint prostheses. Some of thesetheories are described by Dumbleton, J. H., et al., in “The WearBehavior of Ultrahigh Molecular Weight Polyethylene,” Wear, vol. 37, pp.279-289 (1976); Nusbaum, H. J., et al., in “Wear Mechanisms forUltrahigh Molecular Weight Polyethylene in the Total Hip Prosthesis,” J.Appl. Polymer Sci., vol. 23, pp. 777-789 (1979); and Engh, G. A., etal., in “Polyethylene Wear Metal-Backed Tibial Components in Total andUnicompartmental Knee Prostheses,” Journal of Bone and Joint Surgery,vol. 74-B, pp. 9-17 (1992). According to these theories, prosthesescontaining a UHMWPE component in articulating contact a metal or metalalloy component undergo both adhesive and abrasive wear. Material isdisengaged from the surface of the UHMWPE component by asperities of themetal component or by third-body abrasion when previously separatedparticles are drawn across the contact interface. Additional theoriescite the occurrence of surface and subsurface cracking caused by highcontact stresses at the surface. Subsurface cracks propagate through thematerial and join other subsurface and surface cracks, leading todelamination and the deterioration of the delaminated material intoparticulate debris.

The particles released during the wear of UHMWPE components in totalknee replacements are on the order of 1 micron in size. Particles ofthis size elicit an immune response in neighboring tissues. Since giantcells (macrophages) generally do not metabolize such particles, theparticles remain in the physiological system and lead to chronicinflammation and pain. Fatigue due to subsurface cracks may itself leadto catastrophic failure, but fatigue coupled with wear is generally thegreatest life-limiting factor. Debris from frictional sliding betweenthe polymeric and metallic surfaces of the implant leads to clinicalcomplications long before the materials fail due to macroscopic fatigue.

Immune reactions from particulate debris and mechanisms by which thesereactions lead to osteolysis or accelerated bone re-absorption arereported by Schmalzried, T. P., et al., “Polyethylene Wear Debris andTissue Reactions in Knee as Compared to Hip Replacement Prostheses,”Journal of Applied Biomaterials, vol. 5, pp. 180-190 (1994); and Lewis,G., “Polyethylene Wear in Total Hip and Knee Replacement,” Journal ofBiomedical Materials Research, vol. 38, pp. 55-75 (1997). Osteolysisleads to degradation of the anchoring bone, making revision surgery moredifficult if not impossible, as reported by Howie, D. W., “TissueResponse in Relation to Type of Wear Particles Around Failed HipArthroplastics,” J. Arthroplasty, vol. 5 (1990). The effect of particlesentering the lymph nodes is largely unknown.

Other investigators have examined the material properties of the femoralcomponent and have suggested a range of possible alternative materialsand surface modifications, as discussed in Ratner, B. D., et al.,Polymer Surfaces and Interfaces, edited by Feats, W. J., et al., JohnWiley, Chichester, UK, pp. 231-251 (1987); Davidson, J. A., et al.,“Surface Modification Issues for Orthopedic Implant Bearing Surfaces,”Materials and Manufacturing Processes, vol. 7, pp. 405-421 (1992); andWalker, P. S., et al., “Wear Testing of Materials and Surfaces for TotalKnee Replacement,” Journal of Biomedical Materials Research, vol. 33,pp. 159-175 (1996).

Further disclosures of potential relevance to this invention aredescriptions of the use of radio frequency power sources used toenergize a gas to produce a plasma as disclosed in Kolluri, O. S.,“Plasma Surface Engineering of Plastics for Medical DeviceApplications,” Materials Plastics and Biomaterials (1995). The effect ofhigh concentrations of CF₃ groups on the surface of UHMWPE in promotingthe binding of proteins is described by Castner, D. G., et al., “RF GlowDischarge Deposition of Fluorocarbon Films for Enhanced ProteinAdsorption,” Annual Meeting Society for Biomaterials, San Francisco,Calif., p. 218 (Mar. 18-22, 1995).

SUMMARY OF THE INVENTION

It has now been discovered that prosthetic implants with components madeof UHMWPE or other high molecular weight polymers that suffer thedisadvantages enumerated above can be improved by treating the surfaceof the polymeric component with a plasma gas to produce variousconversions or modifications of the polymer at and near the surface. Byappropriate selection of the plasma gas and the conditions of treatment,one can select a particular conversion or modification to address aparticular problem or to benefit the polymeric component and the implantas a whole in any of a variety of ways, such as improving wearresistance, reducing the tendency toward the release of particulardebris, lessening friction between the polymeric component and anadjacent component, increasing either the hydrophilic character or thehydrophobic character of the polymer surface, modifying the chemistry ofthe surface by attaching functional groups, sterilizing the surface,roughening the surface, or making it more biocompatible.

One conversion achievable by the practice of this invention iscrosslinking of the polymer at the surface. This improves the wearresistance of the polymer by reducing or eliminating the tendency of thepolymer chains and the crystalline lamellae to align at the surface andthus reducing their susceptibility to breakage into particles.Conversely, it has been discovered that crosslinking throughout the bulkof the polymer is not beneficial, since it lowers the resistance of thepolymer to crack propagation and thereby renders the polymer componentmore susceptible to fatigue. Crosslinking in a concentrated manner atthe surface, and preferably also in regions near the surface with acrosslinking density that decreases with increasing distance from thesurface, thus improves the wear resistance without substantial loss ofcomponent fatigue resistance.

Other conversions achievable by the practice of this invention, eitherin conjunction with or independent of crosslinking, are couplingreactions between the polymer surface and the plasma gas. Included amongthese reactions are the covalent attachment of groups to the surface,using groups that have particular functionalities or hydrophobic orhydrophilic characteristics that benefit the longevity or utility of thepolymer as a component of the implant, or the compatibility of thepolymer with the surrounding tissue. The plasma reagent may thus be onethat places hydroxyl groups or other hydrophilic groups on the polymersurface, or one that places hydrophobic organic groups or low-frictionfluorocarbon groups on the surface. The lowering of friction achieved bythe covalent attachment of fluorocarbon groups when combined withsurface crosslinking is particularly effective in minimizing sheardeformation, bulk fractures, and surface delamination of the polymericcomponent. This in turn reduces and possibly eliminates the presence ofloose particles, the loosening of joints, and the re-adsorption of bone.

Plasma treatment in accordance with this invention can thus be used tomodify the surface chemistry and microstructure of the polymericcomponent of an implant in ways that will benefit the component and theimplant, and treatments producing two or more effects can be performedsimultaneously or in sequence. The treatments can also be combined withadditional treatments for supplementary purposes such as a preliminarysterilization of the component. These and other features, advantages,and aspects of the invention are described below in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bar graph showing experimental results in terms of wear rateand coefficient of friction, showing the effect of this invention bycomparing test samples of UHMWPE treated in accordance with thisinvention with untreated samples.

FIG. 2 are surface profilometry scans comparing test samples of UHMWPEtreated in accordance with this invention with untreated samples.

FIGS. 3a and 3 b are x-ray photoelectron spectroscopy multiplex scans ofan untreated UHMWPE sample (FIG. 3a) and a UHMWPE sample treated inaccordance with this invention (FIG. 3b).

FIG. 4 is a bar graph comparing friction coefficients of test samples ofUHMWPE treated in accordance with this invention with frictioncoefficients of untreated samples.

FIG. 5 is a bar graph showing the results of biocompatibility tests ontest samples of UHMWPE treated in accordance with the invention anduntreated samples.

FIG. 6 is a plot of x-ray photoelectron spectroscopy data taken fromsurface scans of test samples of UHMWPE treated in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Plasma treatments of a polymeric workpiece in accordance with thisinvention are achieved by placing the workpiece in contact with the gasto be used in the treatment and imposing high-energy radiation,preferably radio-frequency radiation, sufficient to ionize the gas to aplasma state. While not intending to be bound by any particular theoryor mechanism of operation, it is believed that the plasma activates thepolymer chains that are in contact with the plasma by dissociatingcovalent bonds in the polymer chains to form free radicals that arereactive with each other or with free radicals in the plasma gas itself.The reactions that then occur at these activated sites will vary withthe type of gaseous substance used to form the plasma, or with operatingconditions such as the power density, exposure time, working pressure,gas flow rate, temperature, electrode spacing, chamber dimensions,substrate bias voltage, or combinations of these conditions. Theactivation of the polymer by dissociation of its covalent bonds prior tocrosslinking or to a coupling reaction with the plasma gas may beperformed as a preliminary activation step or simultaneously with thecrosslinking or coupling reaction. Likewise, crosslinking may beperformed as a preliminary step to coupling, or crosslinking andcoupling may be performed simultaneously. When the procedure isperformed as two or more steps in sequence, different treatment gasesmay be used for each step, or the same gas may be used but withdifferent operating conditions such as, for example, a stepwise changein power density. The choice of treatment, whether crosslinking, anincrease in the hydrophilic or hydrophobic character, or the coupling offunctional groups of various kinds, will depend on the use contemplatedfor the treated polymer, whether as part of a prosthetic implant, adiagnostic or therapeutic medical device, or as a substrate for proteinsor biological cells in a laboratory or clinical procedure. Appropriatetreatments can be selected for any of various transformations of thesurface, including enhancing the shear strength of the component,lowering the friction coefficient of the component, or rendering thesurface compatible with any of various proteins or other substances thatthe surface will contact when in use or that are to adhere or affix tothe surface.

The gas used may range from gases that are otherwise inert and do notthemselves bond to the polymer to those that are coupling, oxidizing, orreducing reagents and chemically transform the polymer by the additionof groups or atoms. Examples of gases that are useful in activating thesurface polymer chains are the noble gases, hydrogen gas, oxygen gas,organic fluorides and hydrocarbons. Preferred among these are argon,helium, hydrogen gas, oxygen gas, and tetrafluoromethane. Examples ofgases that are useful in converting the surface polymer chains toincrease their hydrophilic character are oxygen gas, acetic acid,volatile siloxanes, ethylene oxide, and hydrocarbons with hydrophilicgroups. Examples of gases that are useful in increasing the hydrophobiccharacter of the surface polymer chains are organic fluorides,particularly trifluoromethane (HCF₃), tetrafluoromethane (CF₄),tetrafluoroethane (C₂H₂F₄), hexafluoroethane (C₂F₆), difluoroethylene(C₂H₂F₂), and hexafluoropropylene (C₃F₆), as preferred organicfluorides, and tetrafluoroethane, hexafluoroethane, andhexafluoropropylene as the most preferred. These species can be usedindividually or as mixtures. Preferred atomic ratios of carbon:fluorinein the treatment gas are in the range of 2:1 to 3:1.

When organic fluorides are used as the treatment gas, it may also bedesirable to include in the treatment gas a fluorine scavenger tocontrol the degree of etching on the polymer surface. Examples offluorine scavengers are hydrogen gas, sulfuric acid gas, methane ormixtures of these gases. Preferred mixtures of gases for use as thetreatment gas are CF₄/H₂, CF₄/CH₄, C₂H₂F₂/CH₄, CHF₃/CH₄, C₂H₂F₄/CH₄,C₂F₆/CH₄, and C₂F₆/CH₄.

The power applied to convert the gas to plasma form will likewise beselected in accordance with the effect sought to be achieved and thedesired depth to which the effect will penetrate below the surface intothe bulk of the polymer. Penetration depths may be less than onemillimeter, or within the range of 1-10 mm or greater. In mostapplications, best results will be obtained using a power density,expressed in terms of wattage per unit area of the surface to betreated, ranging from about 2 to about 100 watts per square centimeter,preferably from about 5 to about 50 watts per square centimeter, andmore preferably from about 3 to about 30 watts per square centimeter.When conversions are preceded by activation of the polymer, typicalpower densities for the preliminary activation step may range from about1 to about 10 watts per square centimeter, preferably from about 2 toabout 5 watts per square centimeter.

Other treatment conditions are likewise variable and are not critical tothe novelty or utility of the invention. The exposure time for examplewill be selected with the considerations similar to those used for thepower density. In most applications, best results will be obtained withexposure times ranging from about 2 minutes to about 60 minutes, andpreferably from about 4 minutes to about 30 minutes. When activation ofthe polymer is performed as a preliminary step, typical exposure timesfor the preliminary activation step may range from about 0.5 minute toabout 20 minutes, preferably from about 1 minute to about 5 minutes. Thepressure in the plasma chamber will likewise be subject to similarconsiderations, with best results generally obtainable at a pressurewithin the range of about 50 mtorr (6.65 pascals) to about 250 mtorr(33.2 pascals), preferably from about 80 mtorr (10.6 pascals) to about230 mtorr (30.6 pascals), and more preferably from about 80 mtorr (10.6pascals) to about 130 mtorr (17.3 pascals). The flow rate of the plasmagas across the workpiece surface being treated may likewise vary,typically from about 50 to about 2000 cubic centimeters per second(measured under standard conditions of temperature and pressure, andexpressed as sccm), and preferably from about 100 sccm to about 1000sccm. Optimal flow rates within these ranges will vary with the size ofthe treatment chamber. The treatment does not require elevatedtemperature and is readily performed at temperatures less than 50° C.,preferably from about 20° C. to about 40° C.

Plasma treatments in accordance with this invention can be combined withplasma treatments for other purposes, such as sterilization of thepolymer surface, removal of contaminants by etching away weakly bondedmolecules, alteration of the surface topography, or increasing surfacebiocompatibility. Sterilization, for example, can be achieved by afive-minute treatment with hydrogen peroxide plasma, which is preferableto conventional sterilization methods such as gamma radiation thatrequire post-processing and cause long-term degradation of the bulkproperties of the polymer. Surface roughness can be altered by etchingaway surface material, and biocompatibility can be increased bytreatment with ammonia.

This invention is applicable to high molecular weight polymers ingeneral that are disclosed for use in the literature, or otherwise knownto be useful, in manufacturing components of orthopedic implants orcomponents of other medical or clinical devices. For artificial knee andhip joints, the polymer that is currently of the greatest interest isultra high molecular weight polyethylene (UHMWPE), particularly thosegrades with molecular weights ranging from about 35,000 to about6,000,000 g/mole, a crystallinity of 0-90%, and a density of about 0.91to about 0.98 g/mL. Further descriptions of this material and similarmaterials are found in Li, S., et al., “Current Concepts Review—UltraHigh Molecular Weight Polyethylene: The Material and Its Use in TotalJoint Implants,” The Journal of Bone and Surgery, vol. 76-A, no. 7, pp.1080-1090 (July 1994), and Kurtz, S. M., et al., “Advances in theprocessing, sterilization, and crosslinking of ultra-high molecularweight polyethylene for total joint arthroplasty,” Biomaterials, vol.20, pp. 1659-1688 (1999). The contents of these papers are incorporatedherein by reference. Other polymers of interest that this invention isapplicable to are high-density polyethylene, medium-densitypolyethylene, low-density polyethylene, polymethylmethacrylate,silicones, and polyurethanes.

As noted above, the plasma is generated by any form of high-energyradiation that will plasma the treatment gas in plasma form.Radio-frequency and ultraviolet radiation are examples; radio-frequencyenergy is preferred.

Although this invention is of broad application, it is of particularinterest in the manufacture of component parts for articulatableprosthetic implants that include a polymeric component with a surfacethat is in sliding contact with a second component that is oftenconstructed of a metallic or ceramic material. An example of such animplant is a knee implant in which the polymeric component whose surfaceis to be treated in accordance with this invention is an acetabular cupover an annular area of a metallic femoral head. The femoral head, whichis generally referred to as a “counter-bearing surface,” may beconstructed of metal, ceramic, or polymeric material which may be thesame or a different polymer than that of the acetabular head. Commonmaterials for the femoral head are ceramics and metal alloys such asCoCr and Ti₆Al₄V. This invention is useful in enhancing the tribologicalcharacteristics of the polymer acetabular head.

The following examples are offered only as illustration and are notintended to limit the scope of the invention.

EXAMPLE 1

This example illustrates the use of plasma treatments in accordance withthis invention in three multi-step treatment protocols on disks ofUHMWPE, using a fluorocarbon plasma in two of the protocols and oxygengas in the third.

Flat, circular disks measuring 3.0 inches in diameter (7.6 cm diameter,45.6 cm² area) were machined from medical-grade UHMWPE (GUR 415, HoechstCelanese), and were polished, then degreased, and finally cleaned byultrasound. The disks were then exposed to various plasma treatments inaccordance with the invention, using 13.56 MHz radio frequency (RF)plasma discharges. The conditions for each treatment are listed in TableI, in which the exposure time for each plasma treatment is expressed inminutes, the power density is expressed as watts per unit area (squarecentimeter) of disk surface, and the flow rate of the treatment gas isexpressed in standard cubic centimeter minute (sccm).

TABLE I Plasma Treatment Conditions Power Gas Flow Treat- Time DensityRate Pressure ment Step Gas (minutes) (W/cm²) (sccm) (mtorr) 1-A 1 Ar 14.4 490 228 2 C₃F₆ 5 8.8 100  88 3 C₃F₆ 1 0 200 129 4 Ar 2 0 490 217 1-B1 Ar 1 4.4 490 227 2 C₃F₆ 5 7.7 100  85 3 C₃F₆ 1 0 200 129 4 Ar 2 0 490217 1-C 1 O₂ 1 6.1 500 244 2 CH₄ 15  8.2 220 156

Friction and wear testing were performed by use of a unidirectionalsliding pin-on-disk apparatus using rounded and polished CoCrWNi alloypins having a radius of 3.28 mm. The apparatus consisted of a turntableto support the test disk. The turntable is rotated at 0.1 Hz and the pinis placed over the turntable in contact with the test disk under acontrolled. Four strain gauges in a Wheatstone bridge configuration,together with suitable amplifier and recorder, are used to measure thestrain on the pin resulting from contact with the revolving disk. Thecoefficient of friction at the disk surface is calculated from thestrain measurements. Prior to testing, the disks were coated with alubricant to approximate the physiological environment of a prostheticimplant in actual use. The lubricant consisted of bovine serumcontaining 0.1% benzamidine, 0.1% trypsin inhibitor, and 0.2% sodiumazide (antibacterial agent), all percents by weight. In each experiment,the total sliding distance was 500 m, the applied mean contact pressurewas 25 MPa, and the sliding speed was 25 mm/s. The experiments wereperformed in a clean laboratory environment at an ambient temperature ofabout 25° C. Wear rates were calculated by dividing the total volume ofworn UHMWPE, as determined from cross sectional surface profilometrymeasurements of the wear track, by the total sliding distance.

The wear rates, expressed as 10⁻¹³ m³/m, and the steady-statecoefficients of friction for disks having undergone treatments 1-A and1-B of Table I, are shown in the bar graph of FIG. 1, in which the wearrates are shown as shaded bars and the coefficients of friction areshown as unshaded bars. The corresponding wear rate and coefficient offriction for a control (untreated) disk are also shown for purposes ofcomparison. Each bar on the graph represents the mean of fourmeasurements, while the error lines indicate the standard deviation. Thecoefficient of friction values are shown as the same (0.12) for disksrepresenting treatments 1-A and 1-B as well as the control disk,indicating that the sliding friction behavior was at most onlymarginally affected by the exposure of the disks to the plasmaenvironment. Some treatments, however, for which the results are notshown in FIG. 1, resulted in a greater coefficient of friction than thatof the untreated (UT) disk, while some treatments resulted in unsteady,stick-slip sliding conditions on the disk. These aberrations were mostlikely due to the presence of a discontinuous surface layer and werecircumvented by modifying the process parameters in the final treatmentstep to those conditions shown in Table I.

The data in FIG. 1 show that wear resistance was greater in diskssubjected to treatment 1-A than in the control disk, while wearresistance was lower in disks subjected to treatment 1-B. This ispossibly attributable to a less uniform hydrophobic layer or one withless cohesion to the substrate as a result of the relatively low powerdensity of treatment 1-B.

Surface profilometry scans are shown in FIG. 2 for one sample oftreatment 1-A and for an untreated (UT) disk. The treatment A scan showsa profound improvement in the wear rate.

Further analyses included measurements of the contact angle of advancingdistilled water droplets on the disk surfaces, as a measure of therelative degree of hydrophobicity or hydrophilicity. The advancingcontact angles for disks representing treatments 1-B and 1-C and for theuntreated control disk are listed in Table II.

TABLE II Contact Angles of Untreated and Plasma-Treated UHMWPE TreatmentContact Angle (degrees) None 112 1-B (Table I: Ar/C₃F₆₎ 148 1-C (TableI: O₂/CH₄) 102

Untreated UHMWPE is a hydrophobic polymer. Table II indicates that thedisk subjected to treatment C, which consisted of the O₂ and CH₄ plasmatreatments, displayed a considerably reduced contact angle, indicatingthat the treatment lowered the hydrophobicity of the disk surface. Thedisk subjected to treatment B, which consisted of the Ar and C₃F₆ plasmatreatments, displayed an increased contact angle, indicating that thetreatment increased the hydrophobicity of the disk surface, which isconsistent with the presence of CF_(x) groups on the surface.

Still further data were obtained by performing x-ray photoelectronspectroscopy scans to obtain indications of the chemical composition ofthe disk surfaces and to identify functional groups bonded to thesurfaces during the treatment. FIGS. 3a and 3 b show the scans for theuntreated disk and a disk subjected to treatment 1-B, respectively.Comparison of these two scans shows that the C 1s peak shiftedconsiderably as a result of the plasma treatment from its otherwisenormal value of 285 eV. The shift is attributed to the charging of thepolymer under the x-ray source. In the scan of the untreated disk, onlythe C 1s (˜287 eV) peak, the O 1s (˜535 eV) peak, and some smallimpurity peaks were visible. In the scan of the treated disk, the F 1speak and complementary F peaks are visible, and the C 1s peak isbroadened, indicating the formation of multiple CF_(x) bonds. This isconsistent with the expectation that carbon atoms become more positivelycharged and therefore have a greater binding energy when bonded to themore electronegative fluorine atoms. The peaks labeled 1, 2, 3, and 4represent —CF₃, —CF₂—, ═CF—, and —CF₂—CF₂—, respectively.

EXAMPLE 2

This example illustrates the use of plasma treatments in accordance withthis invention in both single-step and multi-step treatment protocols ondisks of UHMWPE, using a fluorocarbon plasma preceded by an argon plasmain one of the protocols, the fluorocarbon plasma alone in a secondprotocol and the argon plasma alone in the third and fourth protocols.This example also illustrates how the coefficient of friction can bereduced by imposing a plasma treatment for an extended duration and at ahigher power density.

The materials and equipment of Example 1 were used, and the conditionsof the treatment protocols are listed in Table III.

TABLE III Plasma Treatment Conditions Power Gas Flow Treat- Time DensityRate Pressure ment Step Gas (minutes) (W/cm²) (sccm) (mtorr) 2-A 1 Ar  14.4   500 221 2 C₃F₆  5 8.8   200 104 2-B C₃F₆ 30 11.0   40  67 2-C Ar10 22.0 1,500 150 2-D Ar 10 22.0 1,500 150

Friction tests were performed in the same manner as those of Example 1,and the results are shown in FIG. 4. These results indicate that aplasma treatment with an inert gas (argon) at a high power density (22W/cm²) is more effective in terms of reducing friction than a plasmatreatment with a fluorocarbon at low (8.8 W/cm²) and moderate (11.0W/cm²) power densities, even if the fluorocarbon exposure time isgreater than that of the inert gas.

EXAMPLE 3

This example presents further plasma treatments on UHMWPE in accordancewith the invention and reports the coefficients of friction (COF) of thesurfaces of each of the treated samples. The conditions and results foreach treatment are listed in Table IV below, and the results may becompared to untreated UHMWPE for which the coefficient of friction is inthe range of 0.12-0.2.

TABLE IV Plasma Treatment Conditions and Friction Coefficient (COF)Power Gas Flow COF After Treat- Time Density Rate Pressure Last mentStep Gas (min) (W/cm²) (sccm) (mtorr) Treatment 3-A 1 Ar 1-180 4.4-65.8250-1,500 65-200 0.06-0.13 3-B 1 He 10-30 21.9 200-500 200 0.10 3-C 1 O₂1-10 6.6 500 220-250 2 CH₄ 10-30 6.6-11.0 100-220 100-250 — 3-D 1 Ar 511.0 500 220-230 2 CH₄ 10-30 6.6-11.0 100-500 100-250 — 3-E 1 He with 4%H₂ (by 1 6.6 200 140 volume) 2 mixture of equal 20-30 6.6-8.8 100 90-1150.12-0.17 volumes of (i) CF₄ and (ii) He with 4% H₂ 3-F 1 O₂ 1-106.6-11.0 180-200 100-250 0.14 3-G 1 ethylene oxide 1-10 4.4-11.0 —100-250 0.11-0.18 3-H 1 acrylic acid 1-10 4.4-11.0 — 100-250 0.19 3-Ihexamethyldisiloxane 1-15 4.4-8.8 140 100-400 0.18 3-J 1 Ar 1 4.4250-500 230 2 C₃F₅ 5-30 4.4-11.0 50-300 60-130 0.09-0.16 3-K 1 C₃F₆ 5-304.4-11.0 50-300 60-130 0.09-0.15 3-L 2 acetylene 5 7.7 50 30-50 2 C₃F₆ 58.8 500 80 — 3-M 1 O₂/CF₄(1:1) 10 9.9 500 670 2 C₃F₆ 15 8.8-9.9 50 800.12-0.13 3-N 1 Ar 5 11.0 500 220-230 2 CF₄ 5-30 4.4-11.0 100 90-1200.12

Entries in the COF column that are set forth as ranges represent therange of results for a large number of tests. Comparing the results inthis column with the COF value for the untreated sample in FIG. 4(0.15), it is seen that certain samples of all of the treatmemtsresulted in a lowering of the COF, and in some cases, notably treatments3-A and 3-J, improvement was particularly great.

EXAMPLE 4

This example illustrates the treatment of UHMWPE surfaces to render thesurfaces biocompatible.

Coupons of HOSTALEN® GUR 415 UHMWPE (Hoechst Celanese) measuring 9 mm×9mm×2.5 mm were mechanically polished on one side to a surface finish ofR_(a)=0.1 micron. The polished coupons were then ultrasonically cleaned,degreased, and cleaned with argon plasma at ambient temperature, thenexposed to various plasma treatments as indicated in Table V.

TABLE V Plasma Treatment Conditions Power Gas Flow Treat- Time DensityRate Pressure ment Step Gas (minutes) (W/cm²) (sccm) (mtorr) 4-A 1 Ar 15620 100 203 4-B 1 Ar  5 620 100 202 2 C₃F₆ 25 560 100 104 4-C 1 Ar  5620 100 200 2 CH₄ 25 495 100 102 4-D 1 Ar  5 495 100 180 2 NH₃ 40 495150 190

Following the treatment, the coupons were sealed in gas-permeable bagsand sterilized with a hydrogen peroxide plasma at 400 W for 45 minutes.The coupons were then sealed in air-tight bags until they were testedfor biocompatibility.

Quantitative testing for biocompatibility was performed by the3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT)assay of Mossman, T., J Immunol. Methods, vol. 65, pp. 55-63 (1983).According to the assay procedure, the coupons were embedded in 1% agarin 24-well tissue culture plates leaving only the polished surfaceexposed. One milliliter of Hela cell solution at a concentration of3.4×10⁵ cells/mL was then added to each well and the well contents wereincubated at 37° C. A positive control test was performed bysubstituting latex for the agar, and a negative control test wasperformed by substituting silicone. Additional negative control data wasgathered from wells containing only the agar and from wells containingonly Hela cells. The number of cells remaining after 24 hours was thendetermined by reading the absorbance in all wells by spectrophotometerat a wavelength of 570 nm. The results, including two trials, eachresult representing an average of three coupons, are shown in FIG. 5,which is a bar graph indicating the viability of the Hela cells afterbeing subjected to the various treatments. The graph indicates that noneof the plasma treatments adversely affected the Hela cell population.Each of the plasma-treated coupons performed as well as the untreatedcoupons and as well as the negative silicone control.

Evolution of the surface chemistry was monitored using an x-rayphotoelectron spectrophotometer (XPS) (SSM 660, Physical ElectronicsIncorporated, Eden Prairie, Minn., USA). The C₃F₆-treated samples werescanned at 0, 1, 4, 7 and 10 days in air. The CH₄-treated samples werescanned at 0, 2, 3, 9 and 12 days in air. The XPS results are shown inFIG. 6, which is a plot of the C/F ratio and the C/O ratio as functionsof the number of days of exposure to air. In the Figure, the solid linerepresents the C₃F₆-treated coupons and the dashed line represents theCH₄-treated coupons. The data points demonstrate that C₃F₆ and CH₄treatments evolve with exposure to air. For the C₃F₆ treated samples,the C/F ratio increases over time, indicating a net loss of fluorine onthe surface. More detailed scans (not shown in FIG. 6) indicate the lossof CF₃ groups at early times. The CH₄ samples show an increase in theC/O ratio, indicating a loss of oxygen at the surface. Oxygen is acontaminant of the plasma treatment process. Detailed scans of thesesamples (not shown in FIG. 6) illustrate the loss of C—O and C═O bondsat the surface as a function of time.

The net loss of fluorine on the C₃F₆-treated samples can be attributedto rotation of the hydrophobic CF_(x) bonds into the UHMWPE bulk. Thisappears to happen preferentially to the CF_(x) groups. The loss ofoxygen on the surfaces of the CH₄-treated samples can be attributed tooxygen diffusion in the bulk, a known behavior of UHMWPE.

These examples and the discussion that precedes them are offeredprimarily for purposes of illustration. It will be readily apparent tothose skilled in the art that further variations of the operatingprocedures and conditions and substitutions of the materials can be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for treating a UHMWPE component of anarticulatable prosthetic implant in which said UHMWPE component is insliding contact with a second component of said implant to lower thefriction encountered between contact surfaces of said components duringarticulation of said implant, said method comprising exposing saidUHMWPE component to a substance in a gaseous plasma state at a powerdensity and exposure time sufficient to lower the friction at saidcontact surface of said UHMWPE component by crosslinking, said substancebeing a member selected from the group consisting of a reagent thatimparts to said contact surface an increase in hydrophobic character,said reagent being an organic fluoride, and a reagent that imparts tosaid contact surface an increase in hydrophilic character, said reagentbeing a member selected from the group consisting of oxygen gas, aceticacid, siloxane, ethylene oxide, and hydrocarbons.
 2. A method inaccordance with claim 1 in which said substance is oxygen gas.
 3. Amethod in accordance with claim 1 in which said organic fluoride is amember selected from the group consisting of trifluoromethane,tetrafluoromethane, tetrafluoroethane, hexafluoroethane,difluoroethylene, and hexafluoropropylene.
 4. A method in accordancewith claim 1 in which said organic fluoride is a member selected fromthe group consisting of tetrafluoroethane and hexafluoroethane.
 5. Amethod in accordance with claim 1 in which said organic fluoride ishexafluoropropylene.
 6. A method in accordance with claim 1 in whichsaid substance in a gaseous plasma state is defined as a second plasma,and said method further comprises exposing said UHMWPE component to afirst plasma prior to said second plasma, said exposure to said firstplasma being at a power density and exposure time sufficient to activatesaid UHMWPE at said contact surface.
 7. A method in accordance withclaim 6 in which said first plasma is a member selected from the groupconsisting of noble gases, hydrogen, oxygen, organic fluorides, andhydrocarbons.
 8. A method in accordance with claim 6 in which said firstplasma is a member selected from the group consisting of argon, helium,hydrogen, oxygen, and tetrafluoromethane.
 9. A method in accordance withclaim 1 in which said UHMWPE component has a molecular weight rangingfrom about 35,000 to about 6,000,000.
 10. A method in accordance withclaim 1 in which said power density ranges from about 2 to about 100watts per square centimeter of said surface.
 11. A method in accordancewith claim 1 in which said power density ranges from about 5 to about 50watts per square centimeter of said surface.
 12. A method in accordancewith claim 1 in which said power density ranges from about 8 to about 30watts per square centimeter of said surface.
 13. A method in accordancewith claim 1 in which said exposure time ranges from about 2 minutes toabout 60 minutes.
 14. A method in accordance with claim 1 in which saidexposure time ranges from about 4 minutes to about 30 minutes.
 15. Amethod in accordance with claim 1 in which said exposure of said UHMWPEcomponent to said substance in said gaseous plasma state is performed ata pressure ranging from about 50 mtorr to about 250 mtorr.
 16. A methodin accordance with claim 1 in which said exposure of said UHMWPEcomponent to said substance in said gaseous plasma state is performed ata pressure ranging from about 80 mtorr to about 230 mtorr.
 17. A methodin accordance with claim 1 in which said exposure of said UHMWPEcomponent to said substance in said gaseous plasma state is performed ata pressure ranging from about 80 mtorr to about 130 mtorr.
 18. A methodin accordance with claim 1 in which said exposure of said UHMWPEcomponent to said substance in said gaseous plasma state is performed ata temperature of less than 50° C.
 19. A method in accordance with claim1 in which said exposure of said UHMWPE component to said substance insaid gaseous plasma state is performed at a temperature ranging fromabout 20° C. to about 40° C.
 20. A method in accordance with claim 6 inwhich said power density for said first plasma ranges from about 1 toabout 10 watts per square centimeter of said surface.
 21. A method inaccordance with claim 6 in which said power density for said firstplasma ranges from about 2 to about 5 watts per square centimeter ofsaid surface.
 22. A method in accordance with claim 6 in which saidexposure time for said first plasma ranges from about 0.5 minute toabout 20 minutes.
 23. A method in accordance with claim 6 in which saidexposure time for said first plasma ranges from about 1 minute to about5 minutes.
 24. A method in accordance with claim 6 in which saidexposure to said second plasma is performed at a temperature of lessthan 50° C.
 25. A method in accordance with claim 6 in which saidexposure to said second plasma is performed at a temperature rangingfrom about 20° C. to about 40° C.
 26. A method in accordance with claim6 in which said exposure to said first plasma is performed at a pressureranging from about 50 mtorr to about 250 mtorr.
 27. A method inaccordance with claim 6 in which said exposure to said first plasma isperformed at a pressure ranging from about 80 mtorr to about 230 mtorr.28. A method in accordance with claim 6 in which said exposure to saidfirst plasma is performed at a pressure ranging from about 80 mtorr toabout 130 mtorr.